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The subcommissural ependyma of the third ventricle, the rhomboid fossa, and the central canal of the spinal cord were examined in Tupaia, Saimiri, and Cebus by scanning electron microscopy. In all three species studied, fine filaments, granules, and amorphous masses were found at the surface of the subcommissural ependyma representing the original structures of the Reissner's fibre (RF). Bundles of filaments or net-like structures ultimately forming a compact secretory plate develop in the caudal part of the secretory zone. In Tupaia and Saimiri, the secretion collects in a groove-like depression in the centre of the dorsal ventricular wall. In Cebus two secretory pathways occur in the lateral sulci, which eventually merge together in the aqueduct. In the rhomboid fossa and the spinal cord, the RF is in close contact with the ependyma, and its surface is longitudinally grooved and covered with granular material and cells. The present scanning electron microscopic findings speak in favour of a merocrine type of secretion in the apical region of the ependymal cells. Ciliary movement and flow of cerebrospinal fluid are assumed to be the most important factors regulating the formation of the RF.

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